Papers by Keyword: Undercooling

Abstract: Sn-Ag-Cu alloys are considered one of the most favorable lead-free solder systems. In slowly-cooled eutectic Sn-Ag-Cu alloys, sometimes large primary Ag3Sn or Cu6Sn5 intermetallic compounds (IMCs) form. These IMCs may affect the mechanical properties of solders. However, explanations for the formation of these IMCs are still not clear. This study deals with interrupted tests in order to clarify the nucleation of IMCs in the liquid phase. In this study, Sn-4.41Ag-0.63Cu and Sn-3.30Ag-1.47Cu alloys were prepared. According to the thermodynamic calculation, Pandat, the equilibrium solidification paths are described as follows: Sn-4.41Ag-0.63Cu :L → primary Ag3Sn → binary eutectic (Ag3Sn +Sn) → ternary eutectic; Sn-3.30Ag-1.47Cu :L → primary Cu6Sn5 → binary eutectic (Cu6Sn5 + Sn)→ ternary eutectic. The actual solidification process was different from the estimation from the equilibrium phase diagram. In the case of Sn-4.41Ag-0.63Cu, only Ag3Sn grew as a primary phase in the liquid, while in the case of Sn-3.30Ag-1.47Cu, not only primary Cu6Sn5 but also pseudo-primary Ag3Sn grew in the liquid. Ag3Sn may nucleate easily in the liquid phase, but Cu6Sn5 would not nucleate in the liquid.

Abstract: Sn-Cu-Ni alloy solders have attracted considerable attention from electronic packaging manufacturers and suppliers owing to silver- and lead-free feature and low-cost advantage of the solders. However, there is still a lack of in-depth understanding on composition optimization and microstructure control of the solders. In the present study, the influences of the addition of a minute amount of mixed rare earth La-Ce, in the range of 0.05-0.50wt%, on melting characteristics, undercooling behavior and solidification microstructure evolution of Sn-0.7Cu-0.05Ni-xMRE (x=0.05, 0.10, 0.25, 0.50wt%) alloy solders were investigated. The results show that the addition of rare-earth La-Ce has brought about an obvious decrease of the undercooling for Sn-Cu-Ni series of solders, consequently the growth of primary solidification phase of Cu(Ni)-Sn type intermetallic compound has been suppressed and the β-Sn phase has exhibited a microstructural transition from dendritic grain to equiaxed grain with increasing the amount of La-Ce added to the solder.

Abstract: Traditional casting usually solidifies one homogeneous liquid by removing heat from liquid surface, by which the microstructure solely depends on the heat removal rate for the given alloy composition. The newly developed two-liquid composite simultaneous casting concept raises an alternative to control microstructure. The concept introduces the solidification while mixing technique so that the microstructure depends not only on the heat-removing from liquid surface but also the heat transfer from one liquid to another. The paper reports the experimental validation of the concept by two-liquid casting of Sn-Pb alloys. Reduced grain size, increased microhardness, rougher and more fractal grain surfaces are observed for the alloy solidified by two-liquid casting in comparison of the microstructure obtained by conventional solidification techniques.

Abstract: The glass fluxing technique was used to undercool melts of pure Cu and Ge in the presence of a static magnetic field generated by a superconducting magnet. It was found that the mean undercooling of liquid Cu increased with increasing magnetic field, whereas the mean undercooling of liquid Ge did not show any significant changes with increasing magnetic field. Such a difference in the undercooling behavior can be related to the Lorentz force imposed by the magnetic field, which was larger for liquid Cu because of a larger electrical conductivity than that of liquid Ge.

Abstract: In-situ Laser Scanning Confocal Microscopy observations are presented that assess the influence of undercooling before the initiation of the peritectic transition in a Fe-4.2wt%Ni alloy on the resulting kinetics of the peritectic reaction and transformation. In a series of experiments varying the cooling rate, increasing the cooling rate led to a lower temperature at the L/ interface. The resulting peritectic reaction changed from slow 840m/s - 1500m/s, with limited growth into the  to rapid ~11mm/s with significant growth into . In continuous cooling experiments when the nucleation temperature was low, growth into  was high and the reacting species was observed to propagate along the liquid/delta-ferrite interface at a rate of ~11mm/s. The peritectic reaction rate did not appear to be a function of temperature over a measured nucleation temperature range of 5 K. Conversely, the growth rate of austenite into the delta-ferrite in the first 0.03 seconds was observed to increase from 1.5mm/s to 8mm/s as the measured temperature at nucleation decreased.

Abstract: Phase selection in undercooled melts of Cu-14.8Ge and Cu-18Ge compositions was investigated using the electromagnetic levitation technique in combination with substrate quenching and a drop tube. The results showed that the levitated and gas-cooled samples were all solidified into a microstructure consisting of primary -Cu plus peritectic -Cu5Ge. However, the samples quenched onto a copper substrate showed a segregation-free microstructure in the chilled zone, suggesting direct crystallization of the peritectic -Cu5Ge phase from a highly undercooled liquid. The Cu-18Ge samples quenched onto a glass substrate as well as those solidified in the drop tube also showed a segregation-free microstructure. An analysis of the nucleation kinetics revealed that the -Cu5Ge phase had a larger nucleation barrier than that of -Cu for all accessible undercoolings. It was suggested that phase selection in the undercooled Cu-Ge liquid might be controlled by transient nucleation kinetics.

Abstract: In this article, the liquid-solid interface solidification stability of dilute binary alloy systems during unidirectional (z) solidification was investigated. A new quantitative approach is proposed with (z-δ) as a variable to solve the equation of solute diffusion in the liquid, where δ is the real diffusion layer thickness.

Abstract: Dendrite needles grow from an undercooled melt and their shapes depend on the temperature distribution on the solidification front, which are specified by some parameters such as undercooling, capillary length, diffusivity, convection and kinetic effects. Neglecting the convection and kinetic effects, this study numerically computes the quasi-steady-state integral-differential equation to obtain the shape of a dendrite using solvability condition and investigates the effect of parameters changing the temperature field on the shape of a dendrite. The results reveal that the tip shape enlarges with the decreasing undercooling and increasing capillary length. On the other hand, the increase of thermal diffusivity only slightly reduces the tip radius and shape of a dendrite.

Abstract: In this paper, the thermodynamic and kinetic requirements of heterogeneous and homogeneous nucleation of metallic melting were suggested. Based on the kinetic requirements of nucleation, the mathematic model of wetting angle of heterogeneous nucleation was developed, Based on the wetting angel model, it was predicted that the maximum undercooling of homogeneous nucleation for melts is two thirds of melting temperature. With the wetting angel model, the wetting angles of different catalysts in liquid iron were calculated, and calculation results are in agreement with that of other researchers.